Can I install a wooden subfloor under my Oil Furnace?

My existing oil furnace sits in the basement on the bare cement using four 7 and 1/2" bricks. The supply and return ducts are on the top of the furnace. What are my options for installing a wooden subfloor under my furnace. I know the floor needs to be non combustible but can I install the subfloor and then use a metal covering ? If so then what thickness of metal. If I need to leave it on the bare cement and work around it, then how close can I bring the subfloor up to the existing mounting bricks ? I assume that the CSA document B139 has the answers but I don't have access to it.

Thanks